Dave has questioned the "fall-arrest ability" of the rated 22 kN Aspiring sling used on the DB Tree rope saver. He highlights a very important issue relating to all rope savers - particularly those being made for personal use by individual climbers (see Kong Duck blog). That is, manufacturers (and climbers making home-made rope savers) must ensure there is sufficient energy absorption designed into the system to accommodate a 'safe' fall (or is that an oxymoron).
In contrast to the ART cam-based system, traditional cambium savers (and the DB Tree rope saver) rely on the humble friction hitch. Tied correctly, friction hitches typically outperform mechanical devices.
When using a 2-ring based cambium saver with no length adjustment (eg Timbersaws) the energy generated in the event of a fall is transferred to the climbers' friction hitch. The level of 'fall-arrest energy absorption" is dependent on the type of friction hitch you have tied (download Friction Hitch Report here to make comparisons).
The DB Tree rope saver and the Treemagineers Sirius MultiSAVER are adjustable via the use of an additional friction hitch. DB Tree use the 22 kN rated Aspiring webbing sling to tie theirs and the MultiSAVER utilizes a certifed (EN 795B) Sirius RingLOOP. By adding these additional friction hitches to their systems, DB Tree and the Treemagineers have incorporated another level of energy absorption and safety. In the event of a fall, energy is absorbed by both friction hitches - one on the rope saver and one at your Hitch Climber (or equivalent), thereby increasing the fall-arrest ability of their respective rope savers.
The reality is tree climbers should NOT be in a position where the activation of a 'fall arrest' mechanism is necessary. They achieve this by ensuring there is no slack in their climbing system (ie no rope slack below the knee). Tree climbers are trained to use the Work Positioning method which allows them to work, with their hands free, while hanging in their equipment. ie they always work below their anchor point.
The Fall Arrest method is more commonly used in industrial height safety. For example an arborist hydraulic bucket operator would use the Fall Arrest method NOT Work Positioning.
If we agree there should be no slack in a tree climbers system the logical conclusion is: quoting 'fall arrest' ratings is somewhat redundant.
As an addendum to this story, the Aspiring webbing sling used by DB Tree has generated some debate in the past. To prove the reliability of the sling the DB Tree team drop-tested a sling (using 100 kg weights) that had seen nine months of everyday use by Auckland climber Johno Smith. See the test in the video below or visit Treetools on YouTube to view the full series of DB Tree tests.